In recent years, a distributed power source which represents a solar power generation system spreads in a home. Although the solar power generation system has a merit of being environment-friendly, it also has a demerit of having an adverse influence on the quality of an electric power which an electric power company supplies. Especially, there is a problem that a voltage in a power grid may deviate from a stipulated voltage, by the solar power generation system being introduced to homes (customers) in the power grid. The stipulated voltage means a voltage (i.e., a voltage of the range of 95V to 107V in the case of 100V supply) which the electric power company needs to maintain, and which is defined by Electric Utility Law.
Various methods, such as change of a delivery voltage of a power distribution substation, thickening of an electric wire and revision of a transformer tap, can be considered as a way of controlling a voltage, for example. However, in order to control the voltage, it is necessary to judge which voltage is changed in what manner. On the contrary, there is a method of using sensors installed in the electric wire as a method to know the voltage of the power grid. However, at present, there are very few sensors to the scale of the power grid.
On the other hand, there is known a method called a power flow calculation as a way of calculating a voltage in the power grid and a current value in the power grid. Here, when a voltage distribution needs to be calculated for each predetermined time (for example, 30 minutes), obtaining an absolute value and a phase of the current in each customer or an active power and a reactive power in each customer is required for each predetermined time. However, now, most electric power meters introduced into the customers measure an integral power consumption only once a month and introduction of a smart meter which can measure an electric power value for each predetermined time (for example, 30 minutes) is limited to some customers.
Here, Japanese Laid-open Patent Publication No. 2005-57821 discloses a technique that calculates a load current of each customer from an actual measurement value or standard data generally indicated.
As described above, the actual measurement value of the electric power in the customer is known by only the customer into which the smart meter is introduced. Moreover, even if an electric power value is calculated using the standard data generally indicated, as described in the above-mentioned Japanese Laid-open Patent Publication, the characteristics (for example, the rising of an introductory rate of the solar power generation, etc.) of the customers which change every moment cannot be reflected to the electric power value by such standard data. Therefore, even if the power flow calculation is performed using the electric power value calculated with the use of the above-mentioned standard data, values about the voltage and the current in the power grid cannot be estimated with sufficient accuracy.